Wolters Kluwer Health
may email you for journal alerts and information, but is committed
to maintaining your privacy and will not share your personal information without
your express consent. For more information, please refer to our Privacy Policy.

HIV-1 transmission in utero accounts for 20–30% of vertical transmission events in breast-feeding populations. In a prospective study of 463 HIV-1-infected mothers and infants, illness during pregnancy was associated with 2.6-fold increased risk of in-utero HIV-1 transmission [95% confidence interval (CI) 1.2–5.8] and bacterial vaginosis with a three-fold increase (95% CI 1.0–7.0) after adjusting for maternal HIV-1 viral load. Interventions targeting these novel risk factors could lead to more effective prevention of transmission during pregnancy.

In developing countries, most effective prevention of mother-to-child HIV-1 transmission interventions target intrapartum and breast-milk transmission. These include single dose nevirapine at delivery, short-course antiretrovirals, highly active antiretroviral therapy during breast feeding, breast-milk avoidance, early weaning from breast milk, and infant nevirapine prophylaxis [1–3]. As these interventions become more widely available, in-utero transmission is likely to contribute to an increased proportion of mother-to-child transmission events. Determining factors associated with HIV-1 transmission in utero is an important step toward adapting interventions designed to further prevent infant HIV-1 infection.

A number of maternal, infant, and viral factors have been studied in association with in-utero transmission. Those with the strongest evidence for an association include maternal HIV-1 viral load, antenatal antiretroviral therapy, infant sex, low birthweight, and ascending infections, such as chorioamnionitis [4]. The goal of the current study was to define additional correlates of in-utero transmission within a prospective cohort of HIV-1-infected Kenyan women and infants.

HIV-1-infected pregnant women were followed bi-weekly during pregnancy and at 34–36 weeks of pregnancy began oral zidovudine, which they continued through delivery. Sexually transmitted infections (syphilis, gonorrhea, chlamydia, trichomoniasis) and candida were treated following Kenya National Guidelines. After study completion, slides were evaluated for bacterial vaginosis using Nugent's criteria and blood collected at 32 weeks of gestation was assayed for HIV-1 RNA viral load. Neonatal blood collected at birth was tested for HIV-1 DNA on filter paper specimens and HIV-1 RNA in plasma using PCR [5]. In-utero transmission was defined as a positive DNA or RNA assay at less than 48 h after birth. Mother–infant pairs were subsequently followed up monthly, with infant HIV-1 PCR assays performed every 3 months until 12 months postpartum.

Between 1999 and 2002, 36 000 pregnant women were screened for HIV-1, of whom 31 731 (88%) accepted testing and 4512 (14%) were HIV-1-seropositive. One-third of these women accepted referral to the study clinic and 463 (30%) among these consented to study participation and were followed up through delivery. Of the 463, 88 (19%) women had infants who were infected with HIV-1, 77 (17%) infants died, and 48 (10%) were lost to follow-up. Twenty-nine (33%) infants among the 88 infected infants were HIV-1 PCR positive within 48 h of birth and considered infected in utero. Thirty-seven (42%) infants were HIV-1-uninfected at birth and infected at month 1, thus infected intrapartum or via early breast-milk exposure, and 10 (11%) infants were infected via breast milk after 1 month of age. Twelve (14%) infants did not have specimens at birth and precise timing of infection could not be determined; these infants may have been infected in utero, intrapartum, or early postpartum and were excluded from analyses.

Twenty-nine infants infected in utero were compared with 422 infants who were either infected at other time points or remained HIV-1-uninfected during the 12-month study period. Plasma and cervical HIV-1 viral load were found to be approximately one-half log10 higher for women who transmitted in utero than those who did not (plasma: 5.0 vs. 4.6 log copies/ml; P < 0.001 and cervical: 3.0 vs. 2.4; P = 0.004). Whereas absolute CD4 cell count was not different for women in the two groups, CD4 percentage was lower for transmitting women (19.8 vs. 23.6%; P = 0.01). Women who transmitted were also significantly less likely to have received at least 3 weeks of zidovudine (P = 0.02) and 57% of transmitting women had not completed a 3-week course of antiretrovirals prior to delivery. There was a trend for more female infants to acquire HIV-1 in utero (66 vs. 47%; P = 0.06) and infants infected in utero had significantly lower birthweight (2.9 vs. 3.1 kg; P < 0.001) and gestational age using modified Dubowitz criteria (38.5 vs. 39.3 weeks gestation; P = 0.02).

Antenatal diagnosis of a sexually transmitted infection (gonorrhea, chlamydia, trichomoniasis) was not associated with increased risk of transmission when examined individually or combined into a single variable. However, bacterial vaginosis was significantly more prevalent among women who transmitted in utero: 59% of transmitters vs. 35% of women who did not transmit had bacterial vaginosis in the cohort (P = 0.02). In addition, transmitting women were more likely to have had an AIDS-defining illness during the past 1 year and a greater proportion of women who transmitted in utero reported an illness prior to 32 weeks of gestation, which was characterized by diarrhea, fever, or cough (41 vs. 21%; P = 0.01 and 52 vs. 28%; P = 0.01, respectively).

These factors were assessed in multivariate analyses adjusting for plasma HIV-1 viral load, CD4 percentage at 32 weeks, and duration of zidovudine use (Table 1). Using this model, plasma and cervical HIV-1 RNA levels remained significant predictors of in-utero transmission, conferring a 1.9 and 1.5-fold increase in risk per log10 change in HIV-1 RNA in plasma and cervical secretions, respectively (P < 0.05 for both). Receipt of zidovudine for at least 3 weeks before delivery also remained protective after adjusting for viral load and disease stage [odds ratio (OR) 0.4; 95% CI 0.2–1.0; P = 0.04]. Other independent predictors of in-utero transmission in this model were illness during pregnancy and bacterial vaginosis. Illness during pregnancy was associated with a 2.6-fold increased risk and bacterial vaginosis with a 3-fold increase in the likelihood of transmission in utero (OR 2.6, 95% CI 1.2–5.8; P = 0.02 and OR 3.0, 95% CI 1.0–7.0; P = 0.01, respectively; Table 1).

To our knowledge, neither illness in early pregnancy nor bacterial vaginosis has been reported as a risk factor for transmission in utero. Illness during pregnancy may be contributing to increased transmission by transiently elevating HIV-1 viral load or by causing immune activation and increased CD4 target cells. Women describing fever and other complaints during pregnancy may also be manifesting symptoms of acute HIV-1, characterized by a sharp and transient increase in viral load that could elevate transmission risk substantially. The association between bacterial vaginosis and in-utero transmission may be due to bacterial vaginosis organisms causing upper genital tract infections such as chorioamnionitis, endometritis, and placental compromise. Chronic chorioamnionitis has been associated with increased in-utero transmission and could explain negative results from one large clinical trial designed to treat bacterial vaginosis and prevent mother-to-child transmission [6,7]. As both illness in early pregnancy and bacterial vaginosis are amenable to interventions, these data suggest additional means to target the in-utero period, a period that is likely to increase in its relative significance.

Acknowledgements

Written informed consent was obtained from all study participants. This study received ethical approval from the Institutional Review Boards of the University of Washington and the University of Nairobi.

Data presented previously at the XVI International AIDS Conference in August 2006, Toronto, Canada (abstract number THAC0102).

The authors would like to thank all of the study participants and the dedicated clinic and laboratory staff.

C.F. wrote the article with input from all authors. G.J-S. and D. M-N. designed the parent study and supervised its implementation. J.O. contributed viral load data and provided additional laboratory support. R.B. and D.W. played significant roles in recruitment and follow-up of participants and J.H. made substantial contributions to data analysis.

The present research was supported by NICHD grant HD-23412 and the AIDS International Training and Research Program (AITRP) supported by the Fogarty International Center, National Institutes of Health (D43 TW000007). G. J-S. is an Elisabeth Glaser Pediatric AIDS Foundation Scientist. C.F. was supported by the National Institutes of Health (K23 HD041879)